[MMUSIC] DTLS-SDP and JSEP Conflicts

[Adam Roach <[e-mail]>]

MMUSIC --

[I will be posting a separate message to RTCWEB directing interested 
parties to discuss this issue on the MMUSIC mailing list]

During the IESG review of draft-ietf-mmusic-dtls-sdp, EKR identified 
some conflicts between the procedures in DTLS-SDP and JSEP were 
identified. This note is an attempt to summarize them. I have also made 
an initial proposal, for each conflict, regarding which document needs 
to change, in and which way.

Issue 1 (quoting EKR), which raises a couple of additional sub-issues:

> 1. Assuming I understand this document correctly, it conflicts with
> the guidance in JSEP. Specifically, S 4 says:
>
>     No default value is defined for the SDP 'tls-id' attribute.
>     Implementations that wish to use the attribute MUST explicitly
>     include it in SDP offers and answers.  If an offer or answer does not
>     contain a 'tls-id' attribute (this could happen if the offerer or
>     answerer represents an existing implementation that has not been
>     updated to support the 'tls-id' attribute), unless there is another
>     mechanism to explicitly indicate that a new DTLS association is to be
>     established, a modification of one or more of the following
>     characteristics MUST be treated as an indication that an endpoint
>     wants to establish a new DTLS association:
>
>     o  DTLS setup role; or
>
>     o  fingerprint set; or
>
>     o  local transport parameters; or
>
>     o  ICE ufrag value
>
> This seems to say that if there is no tls-id attribute, then an ICE restart
> (which necessitates a ufrag change) requires a DTLS restart. JSEP isn't
> incredibly clear on this point, but 5.7.3 seems to say that tls-id
> need not be present:
>
>        *  tls-id value, which MUST be set according to
>           [I-D.ietf-mmusic-dtls-sdp], Section 5.  If this is a re-offer
>           and the tls-id value is different from that presently in use,
>           the DTLS connection is not being continued and the remote
>           description MUST be part of an ICE restart, together with new
>           ufrag and password values.  If this is an answer, the tls-id
>           value, if present, MUST be the same as in the offer.
>
> I believe that the first sentence is in error, as we clearly
> can't have JSEP implementations requiring that tls-id be present.
>
>     ...
>     
>     o  If the remote DTLS fingerprint has been changed or the tls-id has
>        changed, tear down the DTLS connection.  This includes the case
>        when the PeerConnection state is "have-remote-pranswer".  If a
>        DTLS connection needs to be torn down but the answer does not
>        indicate an ICE restart or, in the case of "have-remote-pranswer",
>        new ICE credentials, an error MUST be generated.  If an ICE
>        restart is performed without a change in tls-id or fingerprint,
>        then the same DTLS connection is continued over the new ICE
>        channel.
>        
> I think the best interpretation of this is that if tls-id is not present
> (and hence unchanged) then ICE restart does not cause DTLS restart.
> This is also my memory of the consensus in RTCWEB. In any case, these
> two documents clearly must match.


My observations/recommendations:

 1. (Issue 1a) EKR is correct that the first sentence of the bullet from
    JSEP needs to be removed so as to enable interoperation with
    non-JSEP implementations.

 2. (Issue 1b) Additionally the final sentence of that bullet ("If this
    is an answer, the tls-id value, if present, MUST be the same as in
    the offer") conflicts with the definition of tls-id ("the offerer
    and answerer generate their own local 'tls-id' attribute values, and
    the combination of both values identify the DTLS association"). In
    this case, the DTLS-SDP document would appear to be correct (the
    fact that the two parties choose different IDs is integral to the
    mechanism's design), so JSEP needs to change.

 3. (Issue 1c) The crux of the matter: does ICE restart cause DTLS to
    restart? The primary rationale outlined in RFC5245 for restarting
    ICE is changing the destination (IP address or port) of an ongoing
    media stream -- which would commonly involve changing to a different
    physical device. While it would, in theory, be possible to transfer
    the TLS state associated with the connection between devices, this
    is rather cumbersome (and, as far as I know, not generally supported
    by TLS libraries). From that perspective, it is my opinion that the
    DTLS-SDP document is correct that an ICE restart necessitates a new
    DTLS connection; and I conclude that JSEP needs to change.


Issue 2 (quoting EKR):

> 2. S 4 says:
>
>     The mux category [I-D.ietf-mmusic-sdp-mux-attributes] for the 'tls-
>     id' attribute is 'IDENTICAL', which means that the attribute value
>     must be identical across all media descriptions being multiplexed
>     [I-D.ietf-mmusic-sdp-bundle-negotiation].
>
> This is not actually what JSEP requires:
>
>     different categories.  To avoid unnecessary duplication when
>     bundling, attributes of category IDENTICAL or TRANSPORT MUST NOT be
>     repeated in bundled m= sections, repeating the guidance from
>     [I-D.ietf-mmusic-sdp-bundle-negotiation], Section 8.1.  This includes
>
> I suspect this is old text.


(Issue 2) JSEP is aligned with 
draft-ietf-mmusic-sdp-bundle-negotiation-38, while DTLS-SDP does not. 
This is a largely aesthetic decision (although the JSEP/BUNDLE approach 
does save a tiny handful of bytes), but I think changing one document 
(DTLS-SDP) makes more sense than changing two. (I suspect the BUNDLE 
formulation more closely tracks consensus anyway).


/a